1. Field of the Invention
This invention relates to an electric power steering unit to be used for a vehicle and, more particularly, it relates to a field of technology effectively applicable to electric power steering gears to be used for rack and pinion type steering systems.
2. Related Art Statement
In recent years, most vehicles are equipped with a so-called power steering gear, and various type power steering unit, such as a hydraulically or electrically operated type power steering unit has been designed to assist the steering power of the vehicle. In these electric power steering units, as one for applicable to rack and pinion type steering systems, a unit which the steering assisting power is obtained by an electric motor arranged coaxially with a rack-shaft, as Japanese Patent Application Laid-Open No. 8-98451, is known.
Such an electric power steering unit shown in FIG. 4, comprises an electric motor 52 arranged coaxially with a rack-shaft 51 so that the steering assisting power generated by the electric motor 52 is transmitted to the rack-shaft 51 by way of a ball screw mechanism 53. Then, the guiding wheels of the vehicle can be turned by utilizing both the manual steering power of the driver and the steering assisting power.
The rack-shaft 51 is linked to the guiding wheels (which hereinafter may simply be referred to as "the wheels") typically by way of tie rods or knuckle arms arranged at the respective opposite ends thereof and also linked to the steering column 54 that is coupled to the steering wheel (which hereinafter may be referred to as "the handle") by way of a rack and pinion gear so that it may be reciprocatively moved in the horizontal directions of FIG. 4 as the steering operation by the driver. The electric motor 52 has a cylindrical yoke 55 containing coaxially therein a cylindrical armature shaft 56 and a field device 57 and is fed with power from a power supply section 58. The field device 57 comprises magnets 59 arranged on the inner peripheral portion of the yoke 55 and an armature core 60 arranged on the outer peripheral portion of the armature shaft 56. The rotary power generated by the electric motor 52 is transmitted to the rack-shaft 51 as the steering assisting power by way of a ball screw mechanism 53 arranged at the left end of the armature shaft 56 in FIG. 4. Note that the armature shaft 56 is supported at a right side portion thereof by an angular bearing 65 held within housing 61.
The ball screw mechanism 53 has a well known constitution, which comprises a large number of balls 64 arranged between a nut section 62 and a screw section 63 thereof, the nut section 62 being press-fit into and caulked against the armature shaft 56. With this arrangement, the rotary power of the electric motor 53 is transmitted to the rack-shaft 51 by way of the nut section 62, the balls 64 and the screw section 63 to produce axial reciprocative power there, which is then used to assist the steering power.
Further, as shown in Japanese Patent Laid-Open No. 59-50864, there exists housing structure for a power steering unit, in which a yoke portion is formed as a cylindrical part made of iron put between right and left brackets. This type of power steering unit is similar to the above-described conventional one in that a motor is provided coaxially with a rack shaft arranged to pass through the center of the housing and auxiliary steering force of this motor is transferred to the rack shaft through a ball screw mechanism.
On the other hand, wheels may be steered to the limit during a servicing operation or when the steering wheel is turned while the vehicle is standing still. At that time, stoppers (not shown) of the tie rods connected to both ends of the rack shaft 51 are abutted against end portions of the housing 61 or the yoke 55 and movement of the wheels is restricted. In that case, impact force is applied to the housing 61 or the yoke 55 owing to abutting of the stopper, and, sometimes, it becomes such large force as more than several tons. In particular, at the time of servicing operation, a vehicle may be jacked up and wheels may be moved at a speed that can not be attained by ordinary operation of a steering wheel. In that case, very large force is generated in each part of the power steering unit, and accordingly, each component part is designed to be able to endure force of about 10 tons at maximum.
However, in the conventional power steering units, the yoke 55 and the armature shaft 56 are very complex in shape, and the above-described strength condition should be satisfied for such complex shape. As a result, not only the number of production processes is increased, but also high machining accuracy and high quality are required, thus increasing cost. In particular, at a stepped portion, there arises stress concentration. Accordingly, it is necessary to increase the strength by, for example, increasing wall thickness at that portion, and thus, working difficulty becomes large and product weight is increased owing to increase in the thickness.
Further, in the conventional power steering unit having the yoke as a separate part of the cylindrical shape, there exist two coupling portions (faucet coupling portions) between the yoke and the brackets, and accordingly, slight dimensional error tends to affect coaxial property of the housing as a whole.